Certain cardiac potentials are not observed on a routine electrocardiogram due to their small amplitude. Among them are the His-bundle potential and delayed ventricular potentials due to slow conduction in the myocardium. The clinical significance of the delayed ventricular potentials are not well understood. Attempts have been made to record these potentials noninvasively by ensamble averaging but the technique suffers from serious limitations. We have recently designed two low noise electrocardiographic systems to record delayed depolarization potentials in the ST-T segment from the body surface on a beat to beat basis. Our preliminary observations suggests that these potentials are recorded in post infarction patients who show a propensity to develop ventricular tachyarrhythmias. We propose to investigate the delayed depolarization potentials in canine infarctions. At multiple intervals following coronary ligation in dog, attempt will be made to record these potentials and verify them with direct eipcardial and intramural recordings. Ventricular vulnerability to arrhythmias by electrical stimulation techniques will be correlated with the amplitude and duration of delayed potentials. The effect of antiarrhythmic drugs on them will also be investigated. The incidence and characteristics of delayed potentials will be systematically studied in post-infarction patients, in patients with spontaneous high grade ventricular arrhythmias and in patients in whom ventricular tachyarrhthmias could be induced by programmed electrical stimulation. The effects of pharmaceutical agents on delayed potentials will be analyzed and correlated with the effects on ventricular arrhythmias. Patients with and without delayed potentials will be followed for the course of the study to evaluate their clinical significance. Recording of late potentials in the low noise electrocardiogram may predict the propensity to develop malignant ventricular arrhythmias. Further, the recording has the potential of directly identifying malignant "reentrant" versus benign "focal" ventricular rhythms.